1. Field of the Invention
This invention concerns a method of forming a layer of the carbide, nitride and carbonitride of titanium (Ti), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr.), manganese (Mn), tungsten (W), molybdenum (Mo), etc., or solid solution of such element and a matrix element on the surface of a material to be treated by using a fluidized bed furnace, as well as an apparatus used for the method.
2. Description of the Prior Art
Technics for forming a carbide or nitride layer on the surface of a material to be treated by using a fluidized bed have been known, for example, in Japanese Patent Application No. Sho 57-213749 (Patent Application Laid-Open No. Sho 59-107990) and Japanese Patent Application No. Sho 59-108054 (Corresponding to U.S. Pat. No. 4,569,862) previously filed by the present applicant.
The method comprises fluidizing a treating agent comprising a mixture of a powder of refractory material, such as alumina (hereinafter referred to as a fludizing agent), a powder of metal or its alloy containing a carbide- or nitride-forming element (hereinafter referred to as a layer-forming agent), and an ammonium halide salt or a metal halide which is sublimable or vaporizable at or below the treating temperature (hereinafter referred to as an activator) by means of a fluidizing gas, such as argon, embedding a material to be treated in a fluidized bed and forming a carbide, nitride or carbonitride at the surface of the material to be treated. The carbide is formed when the gas of the halide of the carbide-forming element evolves, upon reaction of the layer-forming agent and the gas of the activator reacts with carbon in the material to be treated, and the nitride is formed when the gas of the halide of the nitride-forming element generated in the same way reacts with nitrogen in steels or a nitrogen gas introduced into the fluidized bed.
It is necessary for the activator used in this method to be sublimable or vaporizable at or below the treating temperature in order to prevent the solidification of the treating agent during treatment, thereby maintaining the fluidization. Thus, during long-time use of the treating agent, the activator partially escapes to gradually reduce the surface layer-forming performance. Therefore, the thickness of the carbide or the nitride layer formed may possibly be decreased with the elapse of the treating time. As one of the countermeasures for overcoming such a problem, the present inventors have filed PCT/JP Patent Application No. Sho 86/00360 (U.S. Application Ser. No. 66,483) and now U.S. Pat. No. 4,786,526. Specifically, in this prior application, a powder mixture of a layer-forming agent and activator is charged in a vessel made of porous material, disposed together with a material to be treated in a fluidized bed comprising a fluidizing agent and used for the treatment. When the layer-forming performance of the powder mixture comprising the layer-forming agent and the activator is degraded, the vessel is taken out of the furnace, the powder mixture comprising the layer-forming agent and the activator is replaced with a fresh one and the vessel is returned again into the fluidized bed. In this method, however, the powder mixture of the layer-forming agent and the activator has to be replaced every time when the layer-forming performance is lowered, which necessitates troublesome procedures, such as mixing of both of the powders, removal of the exhausted powder from the vessel, etc., and may often interrupt the coating operation.